Such filters are employed, for example, in selective receivers and in frequency analyzers. Conventional band-pass filters utilizing both active RC technology and passive LC technology have a bandwidth inherent in the magnitude of their components. This makes a change in bandwidth difficult, especially with highly selective filters requiring a precise correlation among their components; this correlation cannot be maintained in the case of a changeover which therefore is generally replaced by a switching among the entire filtering units.
An active selective filter with programmable bandwidth is known from the German magazine ELECTRONIK, Vol. 14 of 16 July 1982, pages 48-50. That filter comprises a BCD/decimal decoder with ten outputs respectively connected to the junction points of a chain of eleven cascaded resistors forming part of an active RC network. Though that circuit arrangement enables the selection of ten different bandwidths, its amplification changes with the bandwidth so that a similarly switchable compensation stage must be combined therewith in order to provide a constant overall amplification.
Another active band-pass filter, described in Vol. 7 of ELECTRONIK 1975, pages 46 and 47, enables variations of its Q-factor and its reasonant frequency but these adjustments entail a change in its basic attenuation, accepted as unavoidable.